Bark extracts and their use in plywood bonding

The research deals with extractants of bark. Park of spruce (Picea shies) was extracted with different solvent and extraction conditions. The extraction solvent always consisted of water and added Na2SO3, SO2 or methanol. Solvent and extraction procedures had great influence on extract yield and the polyphenolic content of extract (Stiasny number). With pure water the extract yield and tannin content of the extract were low. Using different chemicals, such as sulphite or alkali, as mentioned above, polyphenolics with higher yield and satisfactory reactivities with formaldehyde could be extracted. Especially extraction with alkali considerably increased the yield of extracts. In gluing experiments were made three-ply birch plywood consisting of 250 x 250 x 1,5 mm birch veneers or nine-ply birch plywood consisting of 500 x 500 x 1,5 mm birch veneers. No pure tannin based adhesives were used. Extracts were always added to different phenolic resins (so far FXTER A or EXTER B). Plywoods were glued under very normal gluing conditions used in industry. Plywoods were tested as prescribes in Finnish plywood standard SFS 2416. The results showed that up to 20 % of phenolic resins can be replaced by spruce extracts so that the plywoods fulfilled the limits of Finnish plywood standard SFS 2415 and Finnply quality control.